1. Field of the Invention
This invention relates to a waveform table for a music synthesizer, and more particularly to method for building a timbre sample databank for a waveform table so as to store various timbre waveforms for a music synthesizer.
2. Description of Related Art
A music synthesizer using a timbre waveform table to synthesize desired sounds is one of a class of music synthesizers having better capability of tone facsimile. Its synthesizing technology includes extracting a certain length, such as 0.1 second, of an actual sound waveform (W) of a pitch from a music instrument and digitizing it into a set of digital data. The set of digital data with its characterized timbre is stored in a memory to serve as a timbre sample, called a Q sample. When the music synthesizer is desired to play a sound, it plays the Q sample once and repeatedly plays the waveform of the last sound period or the last few sound periods of the Q sample. This repeated waveform unit length of the Q sample is called a QL. This synthesizing technology of a music synthesizer is schematically shown in FIG. 1. In FIG. 1, an actual sound waveform W with a certain pitch is extracted from a music instrument. A Q sample is obtained and stored in the memory of the music synthesizer. A synthesized sound waveform R with a repeated waveform unit length QL is played. In this manner, the QL quality determines the tone quality. According to music theory and experiment results, a good QL should satisfy several conditions as follows:
C1. The QL length must be an integer factor of a basic period of the Q sample. Since the Q sample is played only once, a complete synthesized sound is maintained by repeating the QL. If the QL length is not an integer factor of the basic period of the Q sample, each repeat of QL has a discontinuity at the beginning of each QL. For example, a Q sample for a pitch A4 with a frequency of 440 Hz is to be synthesized and played. This A4 Q sample has a basic period of 1/440, which is about 0.002273 seconds. If the basic period is sampled by a sampling frequency of 44000 Hz, one basic period has 100 sample points. The QL length must be exactly one hundred points or an integer multiple of one hundred points.
C2. The repeated QL must have a waveform that can be repeated with a smooth waveform joint for each repeat without inducing a noise. A natural sound from an instrument has a smooth, continuous wave without noise. If the synthesized waveform is not smooth at the joint, it produces a noise which degrades the sound quality.
C3. The repeated QL must have a waveform that simulates the actual sound waveform so as to obtain a facsimile tone.
C4. In order to simplify the hardware of the music synthesizer and efficiently use the memory to store various Q samples from various pitches of various instruments, each QL length of the Q samples and each Q sample length should have their single fixed quantities.
A synthesized sound should satisfy the above four requirements so as to produce a facsimile tone with good quality. However, it is difficult to simultaneously satisfy all the above four requirements. The difficulty can be see in a conventional process to form a timbre sample in the following descriptions, which includes several steps.
1. A length least common multiple (LCM) of the QL lengths of all various Q samples is obtained so as to satisfy condition C4.
2. In order to satisfy condition C4, a Q sample is obtained by extracting a fixed length, such as 0.1 second, from the beginning of an actual sound waveform. This can be seen in FIG. 2.
3. In FIG. 3, a QL from the last period of the Q sample with a length equal to one basic period is chosen.
4. In FIG. 4, a synthesizer sound waveform R is obtained by playing the Q sample once and repeatedly playing the QL.
In this conventional process, a timbre sample file generally satisfying conditions C1 and C4 is obtained, but it does not satisfy conditions of C2 and C3, resulting in several problems as follow:
1. For a Q sample having a regular waveform for each period, the conventional process with the four steps described above can obtain a high-quality Q sample. However, the waveforms and the periods of natural sounds from the instruments have slowly varying amplitude for each single period. In FIG. 5, in the practical situation, each period of a Q sample has a little variation of waveform and period length. As a QL is taken from the last period of the Q sample and repeatedly played to form a synthesized sound waveform R, the joint for each QL is not smooth, as shown in the lowest plot. This does not satisfy condition C2, and causes noise in the synthesized sound waveform R.
2. According to experiments, a QL length including only one basic period can produce a stable synthesized sound waveform R, but it appears to be a monotone. This can be seen in FIG. 6, where a Q sample exhibits variation of waveform in the actual sound waveform, but the synthesized sound waveform R with a QL length of one period lacks variation. In order to satisfy condition C3, a longer QL is the better, so that a synthesized sound waveform R with a variation similar to that of the original waveform is obtained. However, in this manner, a large difference between the front part and the end part of the chosen QL length may occur, giving rise to a trembling sound that periodically manifests in the synthesized sound waveform R. This also degrades the quality of the synthesized sound waveform R. In other word, a proper QL length needs to simultaneously consider the problems of monotone and trembling effects.